My prior U.S. Pat. No. 4,284,686 explains in some detail that certain diseases of the eye and/or visual deficiencies may be caused by, or may be aggravated by, strong sunlight. The patent notes that radiation at the short end of the visible spectrum, that is, at wavelengths on the order of 400-550 nanometers (nm), seem to create the greatest problems.
In discussing the effect of strong sunlight on the eye, the patent observes that the eye contains two different kinds of photoreceptors, viz., cones and rods. The cones comprise the principal receptors in daylight vision (photopic vision), and the rods constitute the principal receptors in night vision (scotopic visions).
The cones are located generally in the central portion of the retina and permit the recognition of fine detail, presumably because they function largely independently of one another. The cones also permit color vision, i.e., they allow hues and saturation to be distinguished. In the presence of bright light, the eye is most sensitive to radiation at about 555 nm.
In general, the rods are located in the peripheral portions of the retina, there being few if any rods found in the central retina. The rods do not permit the recognition of colors, only shades of gray. Their peak wavelength sensitivity is near 510 nm.
As a corrective measure, the patent provides photochromic glasses that are treated to produce a colored surface layer. The colored surface layer exhibits near-zero transmittance of radiations having wavelengths shorter than a selected cutoff wavelength in the range 440-550 nm. The photochromic feature permits the same spectacles to be used both indoors and outside. To develop the colored layer, the photochromic glass is heated in a strongly reducing atmosphere, such as hydrogen. The exposure is at temperatures in the range of 350.degree.-520.degree. C. for at least 12 hours.
All of the commercially important photochromic glasses are glasses which contain a precipitated microcrystalline silver halide phase. It is this phase which is considered to cause the reversible darkening of the glass under exposure to light. U.S. Pat. No. 3,208,860 (Armistead et al.) provides the basic description of this family of glasses. Subsequent work has resulted in the development of many new families of photochromic glasses exhibiting faster darkening and/or fading response. U.S. Pat. No. 4,190,451 (Hares et al.), for example, provides a description of some recently developed photochromic glasses of this type.
Photochromic glasses exhibiting a fixed color or tint in the undarkened state have also been commercially developed. Such products have included glasses containing conventional glass colorants, as well as glasses wherein coloration is imparted by treatment of the glass after manufacture with a coloring surface treatment, rather than by the inclusion of glass colorants in composition. U.S. Pat. Nos. 3,892,582 and 3,920,463 (Simms) for example, disclose thermal reduction treatments useful for imparting yellow surface colors to photochromic glasses, while U.S. Pat. No. 4,240,836 (Wedding) describes a modified thermal reduction treatment which permits the development of a broader range of colors in this type of glass.
Colored ophthalmic lenses, developed in accordance with the patent teachings, have provided relief for patients having light or glare sensitivity problems. Dye-impregnated plastic lenses have been developed as alternatives. The latter are sometimes referred to as "blockers" since they are stated to absorb all of the light below a certain wavelength.
A major problem with the "blocker" lens is that total absorption of part of the spectrum greatly distorts color perception. This may also occur in the surface colored glass lens with an unduly long treating time. However, the time of the reducing treatment may be adjusted so that a carefully controlled small amount of blue transmission, referred to as a "blue leak", occurs. This provides a less severe distortion of color perception.